Ti-rich and Cu-poor grain-boundary layers of CaCu$_3$Ti$_4$O$_{12}$ detected by x-ray photoelectron spectroscopy

TL;DR

This study uses XPS and EDX to analyze CaCu$_3$Ti$_4$O$_{12}$ ceramics, revealing Ti-rich and Cu-poor grain-boundary layers on cleaved surfaces, with copper existing as monovalent only at these boundaries.

Contribution

It provides the first detailed chemical characterization of grain-boundary layers in CaCu$_3$Ti$_4$O$_{12}$ using XPS, highlighting compositional differences not seen with EDX.

Findings

Grain-boundary layers are Ti-rich and Cu-poor.

Copper exists as monovalent only at the grain boundaries.

Evidences suggest specific grain-boundary structure and formation mechanisms.

Abstract

Cleaved and polished surfaces of CaCu$_3$Ti$_4$O$_{12}$ ceramics have been investigated by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX), respectively. While EDX technique shows the identical CaCu$_3$Ti$_4$O$_{12}$ stoichiometry for the two surfaces, XPS indicates that the cleaved surface with grain-boundary layers is remarkably Ti-rich and Cu-poor. The core-level spectrum of Cu 2$p$ unambiguously shows the existence of monovalent copper only for the cleaved surface. Possible grain-boundary structure and its formation are discussed.